1. Field of the Invention
This invention relates processes utilizing plasmas generated by Radio-Frequency (RF) waves. Particularly, this invention relates processes using RF waves of different frequencies at the same time.
2. Description of the Related Art
Processes utilizing plasmas generated by energy of RF waves, which are hereinafter called xe2x80x9cRF plasma processesxe2x80x9d or xe2x80x9cRF plasma processingxe2x80x9d, are very practical in manufacturing semiconductor devices such as large-scale integrated circuits (LSIs) and display devices such as liquid crystal displays (LCDs). In such processes, RF waves of higher frequencies such as in Very-High Frequency (VHF) i.e. 30 MHz to 300 MHz, have been used more often. This point will be described as follows.
One reason higher-frequency waves are used is on the demand for lowering pressures in processes. The demand of lowering process pressures is much relevant to further upgrade of integration levels, i.e. advancing fineness of circuits. As fineness of a circuit is advanced, product defects would be brought with high probability even when very little foreign particles, e.g. dust, are included. Therefore, it is seriously required to prevent inclusion of foreign particles by lowering pressures during processes.
To lower process pressures is, on the other hand, significant factor from view of enabling formation of finer patterns. For example, as fineness of patterns is advanced, aspect ratios of holes such as contact holes and via holes tend to increase. For forming holes of high aspect ratios, technique of reactive ion etching (RIE) is often used. In the RIE, ions extracted from plasma are made incident onto a substrate. For forming a hole of high aspect ratio by ion incidence, it is required to accelerate ions and make them incident perpendicularly onto the substrate. However, ions would be incident obliquely onto the substrate, because they change their travel directions with collision to gas molecules. This easily takes place in ambience of a higher pressure. As a result of oblique incidence of ions, a large number of ions reach to side walls of the hole, bringing the problem of so called waist-swelling, i.e. bowing shape of the hole.
In plasma chemical vapor deposition where a film is deposited utilizing gas-phase reaction in plasma, a higher pressure increases the probability that activated species produced in the plasma are deactivated by collisions. As a result, a large number of unreacting products so called xe2x80x9cdustxe2x80x9d remain, which may cause pollution of environment in the process chamber easily. While a film is deposited onto the surface of a substrate, side products of low vapor-pressures sometimes evaporate from the surface. When the process chamber is at a high pressure, they may return to the surface of the substrate and stay there, degrading property of the film.
From these points, the demand for more-lowered process-pressures has been strong in this field. The demand for process speeding-up is also still strong for upgrading productivity. For speeding up processes, generally, it is required to increase plasma density, i.e. numeral density of charged particles in plasma. However, to speed up a process and to lower the pressure in the process conflict with each other in a sense. That is, the number of gas molecules is reduced more if the pressure is lowered more, which decreases the plasma density inevitably.
Enhancement of plasma-generation efficiency is realized much important for maintaining plasma density high enough even when pressure is lowered. This point is the background of that a higher frequency has been adopted in RF plasma processes. Concretely, previous processes often adopted frequencies in the MF (Medium Frequency) band, i.e. 300 kHz to 3000 kHz, and in the HF band, i.e. 3 MHz to 30 MHz. However, frequencies in the VHF band have been adopted in many recent processes. A Higher frequency shortens alternation cycles of traveling directions of electrons following an alternate electric field. This increases the probability of electrons to collide with neutral gas molecules, enhancing plasma-generation efficiency.
However, in case a process is carried out utilizing plasma generated by the wave of a higher frequency as in the VHF band, another problem may arise from view of securing high process-property and high process-reproducibility. For example, in plasma processing, ions are often extracted from plasma and made incident onto a substrate as in the described RIE. For the ion incidence, it is required to apply an electric field where its potential drops gradually from the plasma toward the substrate. This electric field generally corresponds to sheath electric field. Sheath electric field is one appearing between plasma and a solid contacting to the plasma. In plasma processing, when a substrate is given floating potential by being insulated from the ground, it is enabled to apply a sheath electric field, which is negative against the plasma potential of plasma (≈0V). This negative sheath electric field extracts ions from the plasma and makes them incident onto the substrate.
However, heightening frequency of a wave for plasma generation brings the problem of weakening the sheath electric field This results from that not only ions but also electrons are difficult to follow the field alternation because the frequency is too high. Ion incidence onto a substrate is often carried out by generating self-biasing voltage at the substrate, which is enabled by applying an RF voltage to the substrate. However, if a higher-frequency voltage such as in the VHF band is applied for generating the self-biasing voltage, sufficient volume of the self-biasing voltage cannot be generated because movement of electros following the field alternation is decreased.
In addition, in such a process as plasma CVD, quantity of ion incidence and level of incidence energy must be maintained within an optimum range because they affect the process. On the other hand, there is another optimum condition in applying the RF wave for the plasma generation, which is often incompatible with the optimum condition of the ion incidence.
Considering these points, recently two different frequencies have been adopted. For example, one for plasma generation is in the VHF band, and the other one for ion incidence onto a substrate is in the HF band that is lower than the VHF band.
From the research by the inventor, however, it turned out that usage of two different frequencies such as in the VHF band and the HF band brings some problems that cannot be disregarded. Concretely, it brings the problem that the plasma cannot be retained well because coupling of RF energy with the plasma becomes insufficient, and the problem that the plasma becomes unstable in its initial state.
This invention is to solve the above-described problems, and brings advantages in RF plasma processing that uses two different frequencies. This invention enables to generate and retain plasma sufficiently and stably. This invention enables to enjoy the merits of the dual-frequency plasma processing. The method of this invention comprises a step for applying an RF wave of a first frequency to discharge space, thereby igniting RF discharge of a process gas at the discharge space, and a step for applying another RF wave of a second frequency to the discharge space with a time lag after igniting the RF discharge. The method of this invention also comprises a step for performing a first stage of impedance matching control by providing impedance optimized for igniting the discharge, when application of the wave of the first frequency is started, and a step for switching the impedance matching control to a second-stage that is performed by providing impedance optimized for stabilizing the plasma. The method of this invention also comprises a step for starting igniting the discharge as impedance matching is performed by providing impedance of a preset value optimized for igniting the discharge, a step for fixing impedance to be provided at the preset value for a preset time, a step for monitoring the reflected wave from the discharge space by a monitor, and a step for carrying out an automatic control of impedance provision so as to minimize the reflected wave from the discharge space, according to the signal from the monitor. The system of this embodiment comprises a controller in which a sequence control program. The sequence control program executes the steps as in the above method.